The use of electric vehicles (EVs) has been promoted in recent years to reduce oil consumption and the emissions of harmful pollutants and carbon dioxide. EVs may include battery powered vehicles, fuel cell powered vehicles and hybrid electric vehicles (HEVs). Commercially available HEVs typically employ a battery and an electrical motor drive system that are sized to optimize the energy efficiency of an internal combustion engine (ICE) and to capture a portion of the kinetic energy generated through dynamic braking by the motor during deceleration. Generally, an electrical motor drive system may include one or more drive units, each consisting of a power inverter and a motor. Multiple electrical drive units can be used to provide four-wheel drive capabilities. The power inverter may function as an inverter to convert a DC voltage to an AC voltage suitable to operate the motor. The power inverter may also function as a power converter when the motor is operating in power generation mode.
Most power inverters in current HEVs operate from a DC voltage source, such as a battery, and thus are referred to as voltage source inverters (VSIs). A typical VSI consists of six semiconductor switches arranged in three pairs connected in parallel with each switch pair connected in series. Such a VSI produces a three-phase AC voltage for powering a three-phase AC motor, where the amplitude of the AC voltage is lower than the amplitude of the DC source voltage. Multiple VSIs may be connected to the same DC source and control multiple motors. A DC-DC converter may be used to increase the amplitude of the output voltages beyond the source voltage to operate the motors at higher speeds.
FIG. 1 depicts an example of a dual electrical motor drive system that may be used in a series configuration HEV or a power-split series/parallel configuration HEV. This drive system consists of a battery, a DC-DC converter, a DC bus capacitor (Cdc), two three-phase VSIs (VSI1 and VSI2), two motor/generators and fourteen switches, S1-S12, Sa and Sb. Each of the switches may comprise power semiconductor devices, such as an insulated-gate-bipolar-transistor (IGBT) and diode in anti-parallel connection or a Metal-Oxide-Semiconductor-Field-Effect-Transistor (MOSFET). An electronic controller (not shown in the figure) based on one or more microprocessors is typically used to control the operations of the electrical motor drive system. The inverter bus voltage, Vdc, is raised to a preferred level that is higher than the battery voltage, VB, by the DC-DC converter. In typical operation, one electric motor is operated as a generator driven by an ICE to power the DC bus through the control of the corresponding VSI, and the other electric motor is operated in the motoring mode to supply a driving force to the wheels of the vehicle. Through the proper control of the DC-DC converter, the battery either supplies or absorbs the difference between the power produced by the generator and the power demanded by the motor to handle the variations in the driving force. During dynamic braking, the motor also operates in regenerative mode to produce an AC voltage which is converted to a high-level DC voltage by the VSI. The high-level DC voltage is then reduced by the DC-DC converter to a level suitable for charging the battery. Accordingly, the battery is charged by the generator and the motor.
The use of VSIs in motor drive systems introduces several drawbacks that make it difficult to meet requirements for cost, volume and lifetime for HEV applications. A VSI requires a very high performance DC bus capacitor to maintain a near ideal voltage source and to absorb large ripple currents typically generated by the switching of the motor currents. Currently available capacitors that can meet the demanding requirements of this application are costly and bulky, and their ripple current capability drops rapidly as the ambient temperature increases. The cost and volume of the DC bus capacitor limits the capability of a VSI to operate in elevated temperature environments. A low-temperature liquid cooling system is therefore needed to operate a VSI in the engine compartment of an HEV. Moreover, the reliability of the VSI is limited by the DC bus capacitor and is further hampered by possible short circuits of the phase legs making up a VSI (such as S1-S2, S3-S4, and S5-S6 in FIG. 1). In addition, as shown in FIG. 2, steep rising and falling edges of the pulse trains in the output voltage, vao, generate high electromagnetic interference (EMI) noises, impose high stress on the motor insulations, produce high frequency losses in the copper windings and iron cores of the motor, and generate bearing leakage currents that erode motor bearings over time.
Many of these problems can be eliminated or significantly relieved by the use of another type of inverter, the current source inverter (CSI). As shown in FIG. 3, a CSI operates from a current source IDC and it does not require any DC bus capacitors. A CSI can tolerate phase leg shoot-throughs, and as shown in FIG. 4, can provide both sinusoid-shaped voltage and current (vao and iam) to the motor. Whereas a VSI produces a voltage pulse train, a CSI generates a current pulse train in each phase output. The current pulse train is generated by turning on and off the switches S1-S6 in the bridge according to a pulse-width modulation (PWM) strategy. The pulsed phase currents are then filtered by a simple filter network of three capacitors, Ca, Cb and Cc. This provides nearly sinusoidal currents as well as nearly sinusoidal voltages to the electric motor. The nearly sinusoidal voltages provided by the CSI are preferable to the pulse train generated by the VSI because they eliminate the problems described above that are associated with the steep rising and falling edges of the VSI pulses. The switches S1-S6 of the CSI should be able to withstand the rated voltage of the CSI in both forward and reverse directions. This generally requires the use of IGBTs with voltage blocking capability in both directions. Alternatively, these switches may be realized by connecting a diode in series with an IGBT or MOSFET that has only forward voltage blocking capability.
Although CSIs offer some advantages over VSIs, they cannot be used as direct replacements for VSIs in HEV applications. Commercially viable HEV energy storage devices, such as batteries and ultracapacitors, are in the form of voltage sources, and thus cannot be used directly as a power source for a CSI. Simply putting an inductor in series with a battery for powering a CSI motor drive has at least two problems: (1) the CSI cannot control the motor current at speeds below a certain point determined by the battery voltage, and (2) the CSI cannot charge the battery during dynamic braking. The difficulties of incorporating energy storage devices into a CSI have so far prevented application of the CSI in HEVs.